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Nashville, Tenn. — May 24, 2016 — Limited indoor space wasn't about to thwart Saint Thomas West Hospital's efforts to retrofit its HVAC exhaust air system for more cost-cutting energy recovery, preheating increased outdoor air and enhancing indoor air quality (IAQ).
Instead of space-consuming conventional indoor HVAC ductwork, the retrofit created return-air distribution through ducts constructed of acoustical panels on the exterior of the Nashville, Tenn.-based hospital.
What now appears as a seven-story-high architectural accoutrement on three sides of Saint Thomas' 43-year-old, seven-story K Tower's exterior, is actually three HVAC exhaust-air distribution risers to new rooftop energy-recovery ventilators (ERV).
In the accompanying photo, the three large beige boxes in the middle of the K-shaped building's roof are plenums (collectors of air) that also aesthetically cover the newly installed energy recovery ventilation (ERV) units from street view. Each leg of the K has ductwork rising up its side from the second floor to the roof. Each riser collects exhaust air from all floors and transports it up to the rooftop ERVs for recovering heat to pre-heat outdoor air the hospital brings in for IAQ standards.
The parent organization of Saint Thomas, St. Louis-based Ascension Health, is the nation's largest not-for-profit hospital network and considers environmental stewardship as important as its health ministry. Therefore the estimated savings for recovering exhaust air energy in the ERVs promises to save the hospital significant annual energy costs with a short payback, according to Don King, CEO, Saint Thomas West and Saint Thomas Midtown, both which are part of Saint Thomas Health's nine-location network.
The project would not have been achieved without the collaborative brainstorming of mechanical contractor Nashville Machine Co., Nashville; project architecture firm, Freeman White, Brentwood, Tenn.; and consulting engineer TME, St. Louis, Mo.
Using acoustical panels manufactured by SEMCO LLC, Columbia, Mo., the trio innovated the external acoustical panel ductwork plan along with fabricating an ingenious support mount system that's completely hidden from the building facade. External conventional sheet metal ductwork and visible support brackets would have detracted from the building. "We didn't want it to look like something bolted onto the side of the building," King said.
The HVAC project was part of a four-year, multi-phase $95 million renovation executed throughout the 2.1 million-square-foot campus of the 550-bed Saint Thomas, which has made the Top 100 Hospital list the last 16 years of both Truven Health Analytics and Modern Healthcare.
Besides environmental stewardship and energy savings, the HVAC retrofit brings the circa 1973 building into current ASHRAE Standard 170 "Ventilation for Health Care Facilities," enhances indoor air quality (IAQ) for patients and employees, and helps assure future high hospital industry ratings.
Previously, return air was simply exhausted outdoors, which was customary in 1970s-style building HVAC design strategies. Now the three 4-inch-thick, 5-by-5-foot-rectangular, R-16 insulated, seven-story-high acoustic panel risers, each deliver 45,000-cfm of return air from the wings to their respective single-wheel desiccant ERV to precondition outdoor air. While supply air continues to be delivered via an existing chase inside the building, the acoustical panel riser is divided internally into two 2.5-square-foot sections that deliver general exhaust and isolation exhaust separately.
Nashville Machine pressure-tested both passageways to assure that no future contamination occurs between isolation and general exhaust. The panels' airtight tongue-and-groove design eliminated any air leak repairs commonly required after conventional ductwork fabrication and installation. Three isolation fans by Twin City Fan, Minneapolis, Minn., were also specified.
The outdoor air and heat recovery, along with the existing variable air volume system supplied by three chillers manufactured by the York div.–Johnson Controls Inc. (JCI), Milwaukee, Wis.; one chiller by Carrier–div. United Technologies, Syracuse, N.Y.; and three boilers by Cleaver-Brooks, Lebanon, Pa., are all controlled by the facility's JCI Metasys building management system (BMS).
The most critical factor in choosing panels over sheet metal ductwork was aesthetics. The panels' custom factory powder-coated sandalwood color matches the building's exterior and blends into the facade's exterior. The panels also have custom factory-designed 12.6-foot lengths, that when combined with a 6-inch wide metal support band, matches up with the building's pre-cast concrete seams that are at 13-foot increments. Thus the panel and building seams match and appear continuous.
"This is the result of close collaboration between manufacturer and our installation team," said Travis Hughes, project manager, Nashville Machine, which had experience using acoustical panels as plenums and ductwork on several past projects.
While the risers only service floors two through seven as they ascend up to their respective rooftop ERV plenum, Nashville Machine fabricated an awning that gives a finished, complementary appearance at the bottom of each riser at 8 feet above exit doorways. SEMCO, which also manufactures custom ductwork, chilled beams, and ERV, active desiccant and dedicated outdoor air systems, supplied the awning metal with a matching powder coated finish and Nashville Machine fabricated it in its sheet metal shop.
The riser suspension system was custom-designed by Nashville Machine engineers to remain unseen and allow for seismic and wind loads. It uses hidden wedge anchors drilled into the precast concrete. Connecting to the wedge anchors are unseen 5-by-5-inch tubular steel stubs that are incorporated into the building-side of the panels. The result is a riser with no visible support structure.
Collecting the exhaust air from each wing riser is a 6 (w) by 12 (h) by 63 (l)-foot plenum also constructed of acoustical panels. The plenum aesthetically appears as a rooftop architectural facade and hides the three packaged ERV units from street-level view.
After recovering heat and moisture, if needed in drier winter time operation, the ERVs' return air is exhausted into another acoustical panel plenum that travels 77 feet in the opposite direction from an additional 90-foot-long extended plenum that collects outdoor air on the opposite side of the tower for ERV conditioning. This separation of exhaust and outdoor air prevents cross contamination.
The alternative to the Saint Thomas project's extended plenum was most likely sheet metal duct with very visible welded supports anchored to the exterior of the duct riser and the building's wall. The sheet metal would have needed painting and insulation, plus SMACNA code-mandated interior supports, because the interior dimensions of any ductwork beyond 4-by-4 feet require structural reinforcement. The ductwork would have required a shop-painted exterior coating needing jobsite touch-up because of the inevitable event of nicking the surface during shipping and installation. Additionally, factory powder coating is superior in longevity, durability, and labor costs compared to site-applied primers and coatings, according to Hughes. "It would have cost the project approximately 20 percent more if insulated sheet metal duct was used instead of the panels even though we have our own sheet metal shop," Hughes said.
Furthermore, when ductwork dimensions approach 5 by 5 feet and larger, the economic law of diminishing returns kicks in. Worker handling/safety, structural steel costs, field-installation time, and other job costs all increase as sheet metal duct becomes larger. With panels however, all of those concerns remain more constant as their easy modular assembly does not change as drastically in proportion to the size of the duct.
Besides reinforcement and structural support, other installation costs must be considered for large duct. Versus panels, large sheet metal ductwork requires more building anchoring, welding, or support fasteners, plus extra labor rigging the ductwork into position. Conversely, panels are engineered for jobsite installation and snap together in a similar fashion to an Erector set.
Once the panels arrived at Nashville Machine, they were assembled into 12.6-foot-long sections, shipped to the jobsite and then hoisted into place using rental cranes ranging from 460 ton for the top floors down to 120-ton cranes for lower floors.
With the trend toward 100 percent outdoor air in the healthcare industry, Hughes sees exterior-mounted acoustical panels ductwork as the wave of the future for older hospital buildings with space restraints for returning air to ERVs. "This is a great way to bring buildings into compliance of updated codes," Hughes said.